Vulcanizable rubber compositions and process



nited States Paten VULCANIZABLE RUBBER COMPOSITIONS AND PROCESS HaroldBoardrnan, Wilmington, Del., assignor to Hercules Powder Company,Wilmington, Del., a corporation of Delaware No Drawing. Application May5, 1955 Serial No. 506,359

12 Claims. (Cl. 26085.1)

This invention relates to the art of rubber, compounding and moreparticularly to an improved rubber composition.

The rubber industry depends almost entirely upon sulfur orsulfur-bearing materials as vulcanization agents. The vulcanization ofrubber in the presence of sulfur is easily controlled and there has beena steady improvement in both the rate of curing and the quality ofvulcanized products produced. This is due in part to the development ofmodern accelerators, activators and other additives which are used inconjunction with sulfur for vulcanizing rubber. In addition to sulfur,selenium and tellurium have been used as vulcanization agents andnitrobenzenes and organic peroxides have also been employed to someextent. With certain rubbers such as butyl rubher, red lead andp-quinone dioxime have been used as vulcanization agents.

Despite the fact that the art of vulcanizing rubber is Well over onehundred years old, vulcanized rubber products from even the most modernand highly developed formulations, are unsatisfactory in certainrespects. Sulfur vulcanized rubber, for example, is wanting in agingcharacteristics, that is, resistance to deterioration of its rubberlikeproperties and discoloration in the presence of oxygen or air, heat andparticularly in conjunction with sunlight. There has been a need in theart for a vulcanizing agent capable of producing vulcanized rubbershaving the advantageous properties of sulfur-cured rubbers but withoutthe disadvantageous properties.

In accordance with the present invention, a vulcanized rubber product isprepared by vulcanizing a composition comprising a rubber selected fromthe group consisting of natural rubber and butala'stic polymers, andfrom about 0.1% to about 10%, based on the weight of rubber, of anunsymmetrical peroxide having the structural formula wherein R is anaryl group and R R R R and R are selected from the group consisting ofhydrogen and alkyl groups of less than-4 carbon atoms. R R R R and R mayall be the same or each may be a diiferent group or any two or more maybe the same or different. The vulcanized product thus prepared issuperior in many respects to'vulcanized rubbers prepared using thevulcanization agents most commonly employed in the art. In particular,the vulcanized products of the invention are superior tosulfur-vulcanized rubbers in their resistance to deterioration uponagingin air or oxygen and particularly in the presence of heat and/orsunlight.

The following examples illustrate the specific embodiments of theinvention and show the effectiveness of the specified peroxidevulcanization agents in the process of this invention. All parts are byweight unless otherwise EXAMPLES 1, 2, 3 AND 4 Rubber compositions wereprepared containing the in- 2 gredients in the proportions indicated inTable I. The compositions were made by first preparing a masterbatchcontaining 1200 parts butadiene-styrene copolymer (general purpose coldrubber designated as GR-S 1500) and 600 parts of furnace black("Philblack 'O). The filler was incorporated into the rubber by thoroughmixing on a standard two-roll rubber mill, the rolls being maintained ata temperature of -1l0 F. The polymer was first allowed to form a band onthe front roll and the furnace black added as fast as practicable. Theblend was then cross-cut and cross-rolled about 12 times each and therubber composition removed. Portions of thefinished masterbatch wereweighed out and the peroxides were added to these portions in theamounts shown in Table I. The peroxides were incorporated into therubber composition in the same manner that the furnace black wasincorporated, i. e., by cross-rolling and crosscutting about 12 timeseach after the peroxides had been added. The sheets thus prepared wereallowed to stand The compositions thus prepared were next vulcanized ina closed multiple cavity mill to form sheets 6 inches square and 0.075inch in thickness. These sheets were cured at 300 F. Table II presentsphysical property data on unaged specimens obtained by standard ASTMmethods using ASTM type C dumbbell specimens in the tensile strength andelongation tests.

Table II Cure Composition ot Time, Min. at 300 F. Ex- Ex- Ex- Example 1ample 2 ample 3 ample 4 Mooney Scorch (MS Rotor) Min. to 10 Point Riseat 250 F 28% 28 52 v 7% 200 150 15 475 500 420 420 Modulus at 200%Elonga- 3O 1, 090 800 880 1, 140 tion, p. S. 1 45 1, 570 800 770 1, 65060 1, 620 770 825 1, 965 90 770 800 2, 190 7% 850 890 740 770 15 2, 2902, 390 2, 2, 450 Tensile Strength at Break, 30 2, 760 2, 880 3, 000 2,790 p. s. 1- 45 2,710 a, 030 2,770 2, 950 60 2, 820 2, 910 2, 830 2, 46090 2, 770 2, 870 2, 7% 680 660 710 810 15 540 660 550 600 UltimateElongation, Per- 30 350 450 440 370 cent 45 280 460 430 280 60 280 460440 230 90 430 440 200 7% 43 44 42 42 15 53 52 51 51 Shore A: Hardness30 60 57 58 60 45 64 57 58 64 60 65 57 58 66 90 56 57 67 7% 43 46 46 4615 43 46 46 46 Bashore Resilience 80 44 46 45 46 45 44 46 45 46 60 43 4646 47 90 46 47 47 3 r EXAMPLES s AND-6 The procedure of the precedingexamples wastollowed in preparing natural rubbercompositions of thefollowing formulations:

-a bove compositions were' nextfivulcanized Fin the vmanner ofthepreceding exampleswand unaged physical properties wereffOUI1w8S--fO1lOWS2 EXAMPLE 7 The procedure of Examples .1 to 4 wasfollowed-in preparing a nitrile rubber composition .from 100 parts-f abutadiene-acrylonitrile emulsion seopolymer containing 26% combinedacrylonitrile' (.Paracril B),S0 parts furnace black and 1.54 partstert-butyl(a,a-dirf1ethylbenzyl) The peroxides of the invention areunsymmetrical peroxides having-the structural formula wherein R is" anaryl "group and R R R R and R are selected from the group consisting ofhydrogen and alkyl groups of less than 4 carbon atoms.

The aryl groups referred to in the above formula may, for example, bephenyl, naphthyl, anthryl, phenanthryl and the like. The aryl groups maycontain alkyl substituents as in the case of methylphenyl, ethylphenyl,propylphenyl; isopropylphenyl, butylphenyl, isobutylphenyl,tbutylphenyl, pentamethylethylphenyl, dimethylphenyl, methylethylphenyl,etc.,-and corresponding alkyl -derivatiVes-Eof .the other aryl groupsmentioned. The term aryl" as used herein i'nclu'des alkaryl groups. Whenan alkyl substituent in an aryl group contains less than 4 carbonatoms,'it may be'the s'ame a's or-diiferentfr'om any of R R R or RAryhgroups'inwhich the alkyl substituents, if any contain less than8carbon atoms are preferred.

In general, the peroxides of this invention are charact'ei'izedby'containing at least 10 carbon'atoms andusually'nofm'ore than about'lflcarbo'n atoms. Specific-peroxides preferred in the invention aremethyl(u,a-dirneth- 'ylb'e'rfzyl)" perokide; ler'tiarybutyl(a,a-dimethylben2y1) fperoxide;=rne thyl(anfldirnethyl-pisopropylbenzyl) peroxidef'andhiethyKa,a dimethylpmethylbenzyl)peroxide.

Theperbitides of' the in'v'e'ntionzire nonvolatile per- "oiidesffiie.'," th'ey'havea vap'orpressure sutfciently low "at rubber compoundingand vulcanization temperatures 'to' prevem substantial loss'fdfuheperoxide by volatilize.- ti'o'nduri'n'g riibiier'compoundingandvulcanization. The "nbnvolatileperoitides of theinven'tion are thusdistin- Z-g'uishedfrom"voiatileperortide's' such as di(tertiary butyl)peroitide. I'nlgne'ralft'he 'peroxi'des'of the invention havebdiling'"points greate'i 'thanaboun150 C. and thus canbeutili2e'd"afrelativelymigh rubber compounding and vulcanizationtemperatures.

' Thep'eroiides of the invention decompose at a moderaterate"undenvulcanization conditions to form alkoxy free'uadicals. Thus,for'fexample, methyl(a,a-dimethylbenzylyfpei oxide dcornpos'es to'form amethoxy free iadicaland"an a,a-dimiethylbe'nzyloxy free radical. Theideco'mpositiondf theperoxides is almost entirely depend- "enfupon'temperature. A particula'r advantageous chara'cteris'tic' oftheperoxides is their stability during rubber eempounuing and "theirreactivity during vulcanization.

"Th'e peroxides lof 'th'e invention can be prepared byiri'etli'ods'lcridwri to (heart. "A particularly convenient .method'invblvespridensation ofthe correspo'nding alco- "hols of 'the" generalformula Ra lake-on all. and hydroperoxides having the general formula RaRc( 3-06H wherein R Rg Rg; R R5 and R' are the same as in the generalformula given'hereinabove. The amount of-alcohol utilized shouldbe' atleast the theoretical calculated amount tor'co'mbinewith a1l"of thehydroperoxide and rpreferably' slightly in 'excess of-this amount. Theconde'nsation of the hydroperoxide with the alcohol is car- -ried out inthe presence ofacatalytic am'ountup to about 0.5%of an acid actingcondensation catalyst based on the weight of the alcoholfanduhetemperature used is'prefmbly between (3. p Toluene 'sulfonic acid can beutilized as' the acid-acting condensation catalyst. The hydropefoxidesand alcohols mentioned above may be "prepared in accordance"with-processes well known The peroxides of the invention in which R Rand R are all hydrogen can be producedconvenicntly by con tactingaralkyl hydroperoxides with dimethyl sulfate in an aqueous alkalinemedium.

I orexample, methy1(a,- dir'nethylbenzyl) peroxide can be produced bydispersing cumene hydroperoxide with rapidstirring in an aqueous Thequantity of peroxide utilized in preparing the rubber compositions ofthis invention will depend to a great extent upon the conditions to beutilized during vulcanization of the composition. In general, thequantity may vary from about 0.1% to 10% based on the weightof' therubber. The preferable amount is from about 0.25% to about 7.5%. Theamount of peroxide used may also vary depending upon the type of rubber,the nature-of the composition, that is, whether an accelerator ispresent, etc., and the properties desired in the product. Thecompounding of rubber" and the 'vul'canization thereof are well knownand the same manipulative techniques may be utilized in practicing thisinvention. However, utilizing the peroxides of this invention,formulation of rubber compositions will be simpler in that certainingredients necessary 'for sulfur vulcanization processes areunnecessary.

The examples have shown the use of various peroxides as rubbervulcanization agents and have illustrated the vulcanization of GR-S,nitrile rubber and natural rubber. The invention is thus applicable tonatural rubber and those synthetic rubbers designated in the art asbutalastic polymers [Marchionna, Butalastic Polymers, Reinhold (1946)].More specifically, the butalastic polymers are defined by Marchionna assynthetic, elastic polymers of a butadiene compound (with or withoutother compounds polymerizable therefrom).

The process of the invention can be applied, for example, to syntheticbutalastic polymers such as those prepared by polymerizing diolefins,halogenated derivatives of diolefins or other substituted diolefins, orby copolymerizing diolefins with other compounds containing vinyl groupssuch as styrene, acrylic acid esters and acrylic acid nitrile. Moreparticularly, the butalastic polymers may be those obtained bypolymerizing any conjugated diolefin such as isoprene, dimethylbutadiene and chloroprene or by copolymerizing isoprene and styrene orisoprene and acrylonitrile. The process of the invention is particularlyapplicable to GR-S, both hot and cold rubbers, nitrile rubber andnatural rubber.

In utilizing this invention, suitable fillers, reinforcing agents,antioxidants, vulcanizers, extenders, plasticizers, softeners,processing aids, as well as other activators and accelerators well knownin the preparation of natural rubber and synthetic rubber compositions,may be employed in accordance with this invention. Certain materialswhen utilized in conjunction with the peroxides of this inventionproduce vulcanized rubber products with greater hardness. Utilizing suchmaterials, a given amount of cure can be obtained using less peroxidethan would be necessary in the absence of these materials. Materialswhich act to produce vulcanizates of higher hardness include furnaceblack, diphenyl guanidine, hexamethylenetetramine, hydrated lime,certain rosin soaps, such as heat-treated rosin soaps, sodium hydroxide,triethanolamine, etc.

Zinc oxide, reinforcing grades of carbon black, and the like, may beemployed as reinforcing agents. Preferably, a mixture of suitablefillers and reinforcing agents is employed to give to the compositionsthe particular properties which may be desired. Thus, a mixture of zincoxide and carbon black may be employed in the manner illustrated by theexamples. Suitable pigments,

suchas ultramarine, Vermilion, or the like, may be employed to impart tothe composition a desired color.

In general, because of the marked stability of the tion is employed, theamount of antioxidant added dur-.

ing'the compounding ofthe present compositions may be increasedsuitably.

The use of the vvarious fillers, antioxidants, and the like hereinbeforementioned is well known, and one skilled in the art will have nodifliculty in arriving at a formulation suitable for a particular need.

Vulcanized products prepared in accordance With the present inventionare of particular value in any application where resistance todeterioration in the presence of heat, oxygen, or sunlight is important.are very valuable in the manufacture of automobile tires and will alsobe found useful in the manufacture of inner tubes, rubber hoses,rubber-lined hoses, foot wear, electrical insulator goods, molded rubberarticles and the like. The improved processing characteristics of rubbercompositions containing the peroxides of this invention as vulcanizationagents, that is, the resistance of such compositions to cure duringmilling, greatly improves and facilitates compounding procedures. Theincreased rate at which rubber compositions containing thesevulcanization agents will cure at customary curing temperatures servesto expedite compounding with consequent savings in time and labor. Theimproved characteristics of the vulcanized product are apparent in theirincreased resistance to discoloration, increased tensile strength,superior hysteresis and electrical properties, increased resistance tooxidation and increased resistance to embrittlement on aging.

What I claim and desire to protect by Letters Patent is:

1. A rubber composition comprising a rubber selected from the groupconsisting of natural rubber and synthetic, rubbery polymers of acompound selected from the group consisting of conjugated diolefins andchloroprene, and from about 0.1% to about 10% based on the weight ofrubber of an unsymmetrical peroxide having the structural formula J lRt- -O0C-Rfl II 1 15 wherein R is an aryl group and R R R R and R areselected from the group consisting of hydrogen and alkyl groups of lessthan 4 carbon atoms.

2. The composition of claim 1 in which the peroxide ismethyl(a,a-dimethylbenzyl) peroxide.

3. The composition of claim 1 in which the peroxide is tertiarybutyl(a,a-dimethylbenzyl) peroxide.

4. The composition of claim 1 in which the peroxide ismethyl(a,a-dimethyl-p-methylbenzyl) peroxide.

5. The composition of claim 1 in which the peroxide is methyl(ax-dimethyl -isopropylbenzyl) peroxide.

6. The process of vulcanizing rubber which comprises heating acomposition comprising a rubber selected from the group consisting ofnatural rubber and synthetic, rubbery polymers of a compound selectedfrom the group consisting of conjugated diolefins and chloroprene, andfrom about 0.1% to about 10% of a peroxide having the structural formulaSuch vulcanizates

1. A RUBBER COMPOSITION COMPRISING A RUBBER SELECTED FROM THE GROUPCONSISTING OF NATURAL RUBBER AND SYNTHETIC, RUBBERY POLYMERS OF ACOMPOUND SELECTED FROM THE GROUP CONSISTING OF CONJUGATED DIOLEFINS ANDCHLOROPRENE, AND FROM ABOUT 0.1% TO ABOUT 10% BASED ON THE WEIGHT OFRUBBER OF AN UNSYMMETRICAL PEROXIDE HAVING THE STRUCTURAL FORMULA